The present invention relates to a control system, such as a system for controlling the air-fuel ratio for an internal combustion engine emission control system having a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a value approximating the stoichiometric air-fuel ratio of the air-fuel mixture for the engine so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an oxygen sensor is provided to sense the oxygen content of exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the oxygen sensor with a reference value, an integration circuit connected to the comparator, a driving circuit for producing square wave pulses from the output sginal of the integration circuit, and an on-off type electromagnetic valve for correcting the air-fuel ratio of the mixture. The control system operates to detect whether the feedback signal from the oxygen sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing an error signal for actuating the on-off type electromagnetic valve to thereby control the air-fuel ratio of the mixture.
Such a feedback control system inherently oscillates due to the detection delay of the oxygen sensor. More particularly, the mixture corrected by the on-off type electromagnetic valve is induced in the cylinder of the engine passing through the induction passage and burned therein, and thereafter discharged to the exhaust passage. Therefore, the time when the oxygen sensor detects the oxygen content of the exhaust gases based on the corrected mixture, the corrective action with the on-off type electromagnetic valve has overshot the desired point. As a result, a rich or lean mixture caused by the overshooting is induced in the engine and the deviation is detected by the oxygen sensor. Thus, the corrective action in the opposite direction will be initiated. After such oscillation of the control operation, the variation of the air-fuel ratio of the mixture will converge toward the stoichiometric ratio. Therefore, the deviation of the air-fuel ratio of the mixture is corrected to the stoichiometric ratio with some delay. Consequently, the desired reduction of the harmful constituents may not be achieved.
On the other hand, it has been found that if the three-way catalyst is exposed to such exhaust gases that the exhaust gas content ratio oscillates periodically with respect to a mean exhaust gas content ratio at a proper period, the catalyst is activated to thereby increase the emission reduction effect.